Base oil viscosity

The base oil viscosity for greases typically used to lubricate rolling bearings lies between 15 and 500 mm2/s at 40 °C (105 °F). Greases based on oils having higher viscosities than 500 mm2/s at 40 °C (105 °F) bleed oil so slowly that the bearing may not be adequately lubricated. Therefore, if a viscosity higher than 500 mm2/s at 40 °C (105 °F) is required because of slow speeds, and for bearings operating at extremely high or low temperatures, extreme care must be taken. Under these conditions, contact the SKF application engineering service.

Base oil viscosity also governs the maximum recommended speed for a bearing. The permissible speed for grease is also influenced by the shear strength of the grease which is determined by the thickener. To indicate the speed capability, grease manufacturers often quote a speed factor, which can be calculated using

A = n dm

where

A

=

speed factor [mm/min]

n

=

rotational speed [r/min]

dm

=

bearing mean diameter [mm]

= 0,5 (d + D)

If the speed factor A exceeds 70% of the recommended limits (table 1), verify the influence of the selected lubricant on the speed limits that are provided under Speeds and check whether the rotational speed is within these limits.